The primary aim of this project is to identify risk factors for the development of symptomatic benign prostatic hyperplasia (BPH). The investigation will be performed with Health Professionals Follow-up Study (HPFS), an ongoing cohort of 51,529 U.S. men aged 45 to 80 years in 1992. The risk factors to be examined in the proposed study include: the influence of ethnic group, endogenous androgen exposure, diet, history of vasectomy, sexual history, body habitus, and co-morbid conditions (particularly diabetes mellitus) on the risk of developing symptomatic BPH. Symptomatic BPH will be defined in the study as a combination of the following criteria; incidence of BPH symptoms (as well as their frequency and severity) elicited by 2-yearly questionnaire; report of enlarged benign prostate detected by rectal examination; and report of prostate surgery for BPH. Much of the data on risk factors will be available through the ongoing 2-yearly collection of exposure information in the Health Professionals cohort. Currently, the HPFS collects detailed data by questionnaire on a large number of variables, including participants' ethnic group, history of vasectomy, body habitus, disease history, smoking habit, and (every four years) diet. The present proposal does not seek additional funds to support this ongoing collection of exposure information. Funding is instead requested for collection and analysis of 4,800 blood samples from the members of the cohort in order to carry out sex hormone measurements. In addition to evaluating endogenous androgen exposure as an independent risk factor for BPH, the study will also investigate ethnic differences in levels of sex hormones (as a hypothesis to explain ethnic variation in the incidence of BPH), as well as dietary and other influences on sex hormone levels (as a hypothesis to explain other observed risk factors for BPH). Secondary aims of the project include studying the natural history of symptomatic BPH (incidence rates of surgery and complications), and the association between BPH and prostate cancer. The large size of the cohort, the prospective design, the high follow-up rate (96% by 1990), the collection of reliable information on exposures and outcomes, combine to make the Health Professionals Follow-up Study a unique opportunity to study the hormonal, nutritional, and behavioral determinants of BPH.